Coatings are applied to the interior of metal food and beverage cans to prevent the contents from coming into contact with the metal surfaces of the containers. Contact of the can contents with the metal surface, especially where acidic products such as soft drinks, tomato juice or beer are involved, can lead to corrosion of the metal container and resulting contamination and deterioration of the contents. Can interiors are typically coated with a thin thermoset film to protect the interior metal surface from its contents. Synthetic resin compositions which include vinyls, polybutadiene, epoxy resins, alkyd/aminoplast and oleoresinous materials have typically been utilized as interior can coatings. These heat-curable resin compositions are usually applied as solutions or dispersions in volatile organic solvents.
The ideal coating should have low extractables to avoid contamination of the contents and should cure rapidly to facilitate can manufacture. The cured coating should be highly resistant to a wide variety of food products, both under storage and processing conditions. The interior coating should be substantially free of blisters and should have good adhesion to the metal surface, both on application and after processing.
Relatively thick films are required to ensure complete coverage of the metal and to protect the metal during drawing and forming operations. This is especially true of coatings applied to metal substrates used to produce the end of a can, where film weights of about 5 to about 9 mg/in.sup.2 are typically required. Side seam coatings, which are also applied as thick films and require blister resistant coatings, have similar performance characteristics. The coatings used for food cans and can ends are generally applied and cured into films on high speed coating lines (e.g., coil coating lines). Modern high speed coating lines require a coating material that will dry and cure without defects within a few seconds as it is heated very rapidly to peak metal temperatures of about 450.degree. F. to about 550.degree. F. (about 230.degree. C. to about 300.degree. C.).
Due to the rapid curing speeds involved, attempts to utilize aqueous coatings on modern coil coating lines have encountered particularly difficult problems in avoiding blistering. Blistering typically occurs as cure temperature passes through the boiling point of water. Blistering becomes more acute as the thickness of the uncured coating layer increases and at higher heating rates and higher peak metal temperatures. All of these factors may be present during application of a coating to can ends on a high speed coating line.
Compositions in which the film producing material is dispersed or dissolved in organic solvents are generally used in coating applications where a relatively thick coating is required. Due to environmental and economic drawbacks associated with the use of organic solvents, however, there is an increasing demand for aqueous-based coatings. In addition to being less expensive than organic solvent-based coatings, aqueous-based coatings minimize the environmental impact of organic solvent release and diminish the need to incinerate curing oven effluents. Unfortunately, under rapid, high temperature curing conditions, currently available aqueous-based coatings do not provide satisfactory performance. There is, accordingly, a continuing need for aqueous-based coatings, which will permit the formation under rapid, high temperature curing conditions of protective films which are substantially free of blisters.